Research Objective Number 21--Data collection in population aging (Biodemography) Understanding causes of these patterns is critical for demographers and public health specialists who interpret the patterns as they relate to, humans, and to biologists interested in the genetic and cellular processes causing aging. This project investigates potential biological causes of late-age mortality 'plateaus' that have been observed in cohort studies of both humans and fruit flies. The following potential causes will be studied using fully-factorial experimental design: a) genetic heterogeneity within populations ('demographic selection') b) changes in reproductive status and population sex ratio. c) changes in population density and environmental quality. d) changes in individual activity level and individual reproductive performance. Mortality patterns will be contrasted using a three-way factorial design involving the following treatment categories: genetically heterogeneous (outbred) populations vs. genetically homogenous populations (crosses between inbred lines); single sex (non- reproducing, constant sex ratio) populations vs. mixed sex (reproducing and variable sex ratio) populations; constant density populations vs. populations where density decreases with age. In concert with these treatments, we will assay activity level and reproductive success in aging flies from the different treatment categories. Associations between these individual- level traits and population-level mortality patterns will begin to address the question of whether mortality plateaus are caused by changes in individual mortality risk, rather than by demographic phenomena.